Polymeric molding is a well-developed field. One broad area of polymeric molding involves introducing a fluid polymeric material into a mold, allowing the polymeric material to assume the interior shape of the mold and to harden therein, and then removing a resultant polymeric article from the mold. Such techniques are commonly known as injection molding, intrusion molding, and others. Solid polymeric articles and polymeric foams can be made using these techniques.
Polymeric foam articles can be produced by injecting a physical blowing agent into a molten polymeric stream, dispersing the blowing agent in the polymer to form a mixture of blowing agent and polymer, injecting the mixture into a mold having a desired shape, and allowing the mixture to solidify in the mold. A pressure drop in the mixture can cause the cells in the polymer to grow. Under some conditions cells can be made to remain isolated in such materials, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes a chemical reaction in the polymer material causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a critical temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide.
Polymeric foam molding is well known. Among the many examples of polymeric foam molding art, U.S. Pat. No. 3,436,446 (Angell) describes a method and apparatus for molding foamed plastic articles with a solid skin by controlling the pressure and temperature of the mold.
Microcellular material typically is defined by polymeric foam of very small cell size. Various microcellular material is described in U.S. Pat. Nos. 5,158,986 and 4,473,665. These patents describe subjecting a single-phase solution of polymeric material and physical blowing agent to thermodynamic instability required to create sites of nucleation of very high density, followed by controlled cell growth to produce microcellular material.
Microcellular molding techniques are described in the patent literature. U.S. Pat. Nos. 4,473,665 (Martini-Vvedensky) and 5,158,986 (Cha et al.) and International Patent Application No. PCT/US99/26192 of Pierick, et al. filed Nov. 4, 1999 and entitled “Molded Polymeric Material Including Microcellular, Injection-Molded, and Low-Density Polymeric Material”, and International Patent Application No. PCT/US98/00773 of Pierick, et al., filed Jan. 16, 1998, published Jul. 23, 1998 (WO 98/31521) and entitled “Injection Molding of Microcellular Material” describe a variety of polymeric molding techniques, systems and molded articles, including microcellular articles.
It is known to introduce reinforcing fibers into polymeric articles, including molded polymeric articles, to improve strength. For example, U.S. Pat. No.5,156,907 (Layden) describes a technique for injection molding fiber-reinforced articles. Injection conditions are controlled to produce a fiber plane orientation within the article which reportedly reinforces the article.
U.S. Pat. No. 6,010,656 (Nomura) describes injection molding of light-weight fiber-reinforced resin products. Fiber-containing pellets are melted and injected into a cavity, then the cavity is opened until its volume is equal to that of the final molded product. A high-strength product reportedly results.
Other examples of patents that describe fiber-containing polymeric articles include U.S. Pat. Nos. 4,692,291 (Angell), 5,294,461 (Ishida) and 4,340,562 (Gross). This list is intended to be exemplary, not exclusive.
Although introduction of reinforcing fibers into molded polymeric articles is known, one problem that can be encountered in the injection molding of polymeric articles including reinforcing fibers is that the fibers can break during introduction into the mold, which can compromise properties of the resultant articles. Accordingly, it would be advantageous to produce injection-molded fiber-reinforced polymeric articles (or mold such polymeric articles using similar techniques) while maximizing properties of the final product.